1. Field of the Invention
This invention relates to the electrolytic reduction of aluminum in a Hall cell. More particularly, this invention relates to improvements in a Hall cell which permit more efficient operation thereof.
2. Background Art
In the normal operation of a Hall cell, spacing between the anode and the cathode is adjusted to provide sufficient power consumption to not only reduce the alumina to aluminum but to generate sufficient heat to maintain, in a molten state, the salt bath in which the alumina is dissolved.
However, due to erosion of the carbon lining cathode walls of the bath by reaction with the molten salt or the molten aluminum, as it is formed, the cell is conventionally operated at a temperature which permits solidification of a certain amount of the bath on the carbon sidewalls of the cell. This frozen bath lining, then, acts as a protective liner to prevent interaction between the carbon sidewalls and the molten portion of the salt bath.
Due to the ever increasing costs of electricity and the concurrent need to conserve energy resources, there has been an increased interest in raising the efficiency of the Hall cell operation. It has long been known that a reduction in the spacing between the anodes of the cell and the cathode bottom wall would reduce the power consumption (I.sup.2 R) of the cell. However, a Hall cell does not operate in a quiescent state, and the movement of the molten aluminum in the cell during normal operation could result in shorting out of the cell if the spacing was reduced.
More recently, however, relatively inert conductive cathode materials have been developed which may be used over the carbon cathode bottom wall, for example, in particulate form as a layer spread on top of the carbon cathode bottom wall of the call, to effectively extend the cathode upward toward the anode and thus reduce the anode-cathode spacing. Such materials, which include TiB.sub.2, TiN, ZrB.sub.2 or NbB.sub.2, may also be used in shaped forms such as plates or the like. When used in such a form, openings are provided through which the molten aluminum may flow so that the inert cathode material, not the molten aluminum, is spaced closest to the anode.
While such an approach is, indeed, satisfactory for the reduction of power consumption in a Hall cell, a concurrent problem has arisen with regard to maintenance of the frozen bath protective lining on the carbon sidewalls of the cell. This is because the reduced power consumption of the cell results in less heat generated so that if sufficient heat is removed from the cell through the sidewalls to permit the formation of a frozen bath lining, as in the prior art operation of the cell, the temperature of the cell may be lowered to a dangerous point wherein the entire cell may freeze over.
It, thus, would be desirable to provide a Hall cell having a closer spacing between the anode and cathode, to thereby reduce the amount of power consumed, while still protecting the sidewalls of the cell from erosion and without endangering the operation of the cell due to cell freeze-up.
To compensate for the absence, or substantial absence of a protective layer of frozen bath formed over the carbon cathode sidewall, a liner could be used over the carbon cathode sidewall which would protect the carbon from direct contact with the molten aluminum. Unfortunately, however, candidate materials for such a liner which are capable of resisting reduction by contact with the molten salt bath are usually oxidized at the interface at the top of the bath, i.e., the interface between the molten bath and the frozen crust over the bath.